The present invention relates to a method of manufacturing a spring steel having improved in relaxation resistance at a high temperature. The spring steel is particularly useful as a diaphragm spring in the clutch of a motor vehicle to be operated at a high temperature.
In recent years, springs used in machines are exposed to a higher temperature environment which is caused by an increase of the output power and/or the scale enlargement of the machine. For instance, a diaphragm spring incorporated in a clutch or the like was, at one time, heated up to 150.degree. C. at the highest. As a torque applied to a clutch becomes larger due to an increase of the engine power of a motor vehicle, such as a four-wheel drive vehicle, the environmental temperature of the clutch has become raised up to 250.degree.-350.degree. C., i.e. a warm temperature range from a metallurgical point of view.
A steel spring, which exhibits sufficient property at ambient temperatures, is easily relaxed when it is exposed to such a high temperature environment. Consequently, the property necessary for the spring to act in its intended capacity is rapidly deteriorated. In this regard, the demand for the property of the spring becomes severer. Such demand is not only for a diaphragm spring in the clutch of a motor vehicle but also for other disc springs to be incorporated in other machines with as a function of the increase of output power.
A carbon tool steel such as SK5 (JIS:Japanese Industrial Standard) has been used as the material of a diaphragm spring in a conventional motor vehicle, since it has sufficient relaxation resistance at a temperature below 150.degree. C. However, a diaphragm spring made of the carbon tool steel is rapidly relaxed and becomes inoperative when the environment in which the spring operates is heated to a high temperature (such as 250.degree.-350.degree. C.). Consequently, there has risen a requirement for the development of a steel which exhibits excellent relaxation resistance sufficient to operate at an elevated temperature. In addition, the steel shall have a fatigue strength which is high enough to endure the repetition of applications of load without breaking.
It is well known that an increase of Si content is effective for improving the relaxation resistance of the steel material. For instance, a high-Si steel such as SUP6 or SUP7, defined in JIS G4801, has been used as a spring material which is required to have substantial relaxation resistance. On the other hand, it is known that the quenchability of steel is enhanced by the addition of Mo in combination with increases of the Si and Mn contents, as disclosed in Japanese Patent Publication Laid-Open 2-240240.
Steels such as SUP6 and SUP7, containing large amounts of Si, exhibit excellent relaxation resistance at ambient temperatures. However, their relaxation resistance is reduced with an increase of the environmental temperature, so that a spring made of such steel does not exhibit sufficient property at the elevated temperature (250.degree.-350.degree. C.).
By the way, a quenching-tempering treatment has been applied to a spring steel to obtain the property necessary for its use in springs. If the steel material is held at a high temperature for a long time in advance of quenching in this heat treatment, it is decarburized and oxidized at its grain boundaries. As a result, the fatigue strength of the obtained steel spring is lowered, so that the steel spring would not bear the repetition of load. In order to maintain the fatigue strength at a proper level, carbides need to be rapidly dissolved in the matrix which is being transformed into an austenitic state during the heat treatment.
The relaxation resistance of steel is also improved by heat treatment, e.g. quenching, tempering, applying plastic strain, and then strain aging at 250.degree.-350.degree. C. However, since this heat treatment requires a number of steps, manufacturing costs of steel springs become higher. In this consequence, there is a need for a steel material which, as such in a quench-tempered state exhibits excellent relaxation resistance at high temperatures.